In the electrophotographic process, an electrostatic latent image formed on a photoreceptor is developed with a developer containing a toner, and a toner image obtained is transferred to a recording medium such as paper, followed by fixing with a heat roll, so as to obtain an image on the recording medium. A full color image can be obtained by overlapping toner images of four colors, i.e., cyan, yellow, magenta and black, utilizing the electrophotographic process.
In recent years, an image forming apparatus of an intermediate transfer type has been subjected to practical use, in which toner images of cyan, yellow, magenta and black formed on a photoreceptor are primarily transferred on an intermediate transfer material, and a multi-color image formed on the intermediate transfer material is secondarily transferred to a recording media, and thus speeding up of a color duplicator using the electrophotographic process is realized. In the image forming apparatus of an intermediate transfer type, however, because the number of times of transfer is large, and the multi-color toner image is transferred to a recording medium at a time, it is liable to cause a problem of image defects, such as transfer unevenness and drop-off. Furthermore, it is liable to cause another problem of lack of general purpose property in that even though a good image can be formed on a recording medium having a smooth surface, such as coated paper, image defects occur when an image is formed on a recording medium having a rough surface, such as recycled paper.
JP-A-5-34979, JP-A-5-88409, JP-A-5-197193 and JP-A-5-341573 disclose that the transfer property of the toner is improved by providing fine holes or protrusions on the surface of toner particles having a substantially spherical shape.
In the toner using the toner particles having fine holes and protrusions on the surface thereof, however, there is a problem in that the effect of an external additive coated on the toner particles cannot be sufficiently exhibited.
The external additive fundamentally exhibits such a function in that it intervenes between an image holding member and the toner particles to prevent the toner particles from adhering on the image holding member, whereby the transfer property of the toner is improved. However, in the case where unevenness is present on the surface of the toner particles, when the toner is stirred and subjected to mechanical stress in a developing device, the external additive fails at the protruded surface of the toner particles, whereas the external additive is accumulated at the reentrant surface. Thus, the toner particles are in contact with the image holding member without the external additive intervening at the protruded surface, so as to prevent the external additive from exhibiting the function thereof, whereby image defects, such as transfer unevenness and drop off, occur.
With the progress of decrease in particles size of the toner for realizing a high quality image, it is becoming difficult to completely transfer the toner particles without remaining on the image holding member. The conventional apparatus for forming an image is designed without sufficiently considering the difference in transfer property depending on the size of the toner particles.